I. Field of the Invention
This invention relates to a process for producing lubricating oil from a Fischer-Tropsch wax. More particularly, it relates to a process utilizing a Group VIII metal-on-alumina catalyst for hydroisomerizing a hydrotreated Fischer-Tropsch wax to produce a lubricating oil having a high viscosity index and a low pour point.
II. Description of the Prior Art
In the Fischer-Tropsch process a synthesis gas (CO+H.sub.2) made, e.g., from natural gas, is converted over a catalyst, e.g., a ruthenium, iron or cobalt catalyst, to form a wide range of products inclusive of gaseous and liquid hydrocarbons, and oxygenates, and a normally solid paraffin wax which does not contain the sulfur, nitrogen or metals impurities normally found in crude oil. It is generally known to catalytically convert the paraffin wax, or syncrude obtained from such process to lower boiling paraffinic hydrocarbons falling within the gasoline and middle distillate boiling ranges.
Paraffin waxes have been isomerized over various catalysts, e.g., Group VIB and VIII catalysts of the Periodic Table of the Elements (E. H. Sargent & Co., Copyright 1964 Dyna-Slide Co.) Certain of such catalysts can be characterized as halogenated supported metal catalysts, e.g., a hydrogen chloride or hydrogen fluoride treated platinum-on-alumina catalyst as disclosed, e.g., in U.S. Pat No. 2668,866 to G. M. Good et al. In the Good et al. process a partially vaporized wax, such as one from a Fischer-Tropsch synthesis process, is mixed with hydrogen and contacted at 300.degree. C. to 500.degree. C. over a bed of supported platinum catalyst. Palladium or nickel may be substituted for platinum. The support may be a number of conventional carrier materials, such as alumina or bauxite. The carrier material may be treated with acid, such as HCl or HF, prior to incorporating the platinum. In preparing the catalyst, pellets of activated alumina may be soaked in a solution of chloroplatinic acid, dried and reduced in hydrogen at 475.degree. C.
U.S. Pat. No. 2,817,693 discloses the catalyst and process of U.S. Pat. No. 2,668,866 with the recommendation that the catalyst be pretreated with hydrogen at a pressure substantially above that to be used in the process.
U.S. Pat. No. 3,268,439 relates to the conversion of waxy hydrocarbons to give products which are characterized by a higher isoparaffin content than the feedstock. Waxy hydrocarbons are converted at elevated temperature and in the presence of hydrogen by contacting the hydrocarbons with a catalyst comprising a platinum group metal, a halogenatable inorganic oxide support and at least one weight percent of fluorine, the catalyst having been prepared by contacting the support with a fluorine compound of the general formula: ##STR1## where X is carbon or sulphur and Y is fluorine or hydrogen.
U.S. Pat. No. 3,308,052 describes a hydroisomerization process for producing lube oil and jet fuel from waxy petroleum fractions. According to this patent, product quality is dependent upon the type of charge stock, the amount of liquid hydrocarbon in the waxy charge stock and the degree of conversion to products boiling below 650.degree. F. The greater the amount of charge stock converted to material boiling below 650.degree. F. per pass the higher the quality of jet fuel. The catalyst employed in the hydroisomerization zone is a platinum group metal catalyst comprising one or more platinum, palladium and nickel on a support, such as alumina, bentonite, barite, faujasite, etc., containing chlorine and/or fluorine.
In U.S. Pat. No. 3,365,390 a heavy oil feed boiling at least partly above 900.degree. F. is hydrocracked and the oil effluent thereof is separated into fractions, including a distillate fuel and a higher boiling hydrocracked lube oil boiling range fraction. The hydrocracked lubricating oil boiling range fraction is dewaxed to obtain a hydrocracked wax fraction which is hydroisomerized in the presence of a reforming catalyst and the oil effluent thereof is separated into fractions, including a distillate fuel and an isomerized lube oil boiling range fraction.
In U.S. Pat. No. 3,486,993 the pour point of a heavy oil is lowered by first substantially eliminating organic nitrogen compounds present in the oil and then contacting the nitrogen-free oil with a reforming catalyst in a hydrocracking-hydroisomerization zone. Hydroisomerization is conducted at a temperature of 750.degree..pi.F.--900.degree. F. over a naphtha reforming catalyst containing no more than two weight percent halide.
U.S. Pat. No. 3,487,005 discloses a process for the production of low pour point lubricating oils by hydrocracking a high pour point waxy oil feed boiling at least partly above 700.degree. F. in at least two stages. The first stage comprises a hydrocrackingdenitrofication stage, followed by a hydrocrackingisomerization stage employing a naphtha reforming catalyst containing a Group VI metal oxide or Group VIII metal on a porous refractory oxide, such as alumina. The hydrocracking isomerization catalyst may be promoted with as much as two weight percent fluorine.
U.S. Pat. No. 709,817 describes a process which comprises contacting a paraffin hydrocarbon containing at least six carbon atoms with hydrogen, a fluorided Group VIB or VIII metal alumina catalyst and water. These catalysts are classified by the patentee as a well-known class of hydrocracking catalysts.
III. Summary of the Invention
A process for producing a lubricating oil having a high viscosity index and a low pour point from a Fischer-Tropsch wax which process comprises:
(a) contacting the Fischer-Tropsch wax with a hydrotreating catalyst and hydrogen to reduce the oxygenate and trace metal levels of the wax and to partially hydrocrack/isomerize the wax;
(b) contacting the hydrotreated Fischer-Tropsch wax from step (a) with hydrogen in a hydroisomerization zone in the presence of a fluorided Group VIII metal-on-alumina catalyst having (i) a bulk fluoride concentration ranging from about 2 to 10 weight percent, wherein the fluoride concentration is less than about 3.0 weight percent at the outer surface layer to a depth less than one one hundredth of an inch provided the surface fluoride concentration is less than the bulk fluoride concentration, (ii) an aluminum fluoride hydrate level greater than 60 where an aluminum fluoride hydrate level of 100 corresponds to the X-ray diffraction peak height of 5.66.ANG. for a Reference Standard;
(c) fractionating the effluent from step (b) to produce a lubricating oil fraction boiling at atmospheric pressure above about 640.degree. F., preferably above 700.degree. F.; and
(d) dewaxing the lubricating oil fraction from step (c) to produce a dewaxed lubricating oil having a viscosity index of at least 130 and a pour point less than about 0.degree. F.
In preferred embodiments, the hydrotreating catalyst will be unsulfided, the catalyst employed in the hydroisomerization zone will be a fluorided platinum-on-aluminum catalyst, and the isomerate is contacted with hydrogen in the presence of a hydrogenation catalyst to reduce unsaturation of the isomerate and thereby improve its daylight and oxidation stability.